Photomasks, commonly referred to as masks or reticles, are used in the manufacture of semiconductor integrated circuits (ICs). Typically, a photomask is made of an opaque patterned layer applied on a transparent substrate. By exposing the photomask to a particular wavelength of light in a photolithographic process, an image is projected through the photomask onto a semiconductor wafer creating an imprint of the patterned layer on the wafer.
Since a photomask is used to project this image and any defect on the surface of the photomask will result in a transferred imperfection onto the wafer, any defect on the surface of the photomask could possibly render the wafer useless. Therefore, manufacturers of semiconductor ICs take great care to protect the quality of photomasks used in the manufacturing process. To further such quality, pellicles have been developed to protect the image plane of the photomask.
A pellicle provides a protective membrane over the photomask to keep contaminants away from the image plane of a patterned layer on a photomask. Generally, pellicles are designed to be highly transmissive to particular wavelengths. The use of a pellicle with an incorrect wavelength may cause the pellicle to degrade, which can reduce the amount of light transmitted through the photomask and affect a pattern projected on a wafer. Therefore, manufacturers take care in matching the correct pellicle with the correct photomask for a particular exposure wavelength. Even if a pellicle and a photomask are correctly assembled, a problem may still exist if a photomask assembly, which includes a photomask and a pellicle, is placed in the wrong lithography tool.
A lithography tool, such as a stepper, scanner, or proximity printer, may use a variety of different wavelengths. Although most photomasks may be used with various photolithographic tools, a pellicle membrane is typically wavelength specific. For example, I-line reticles within a device set are assembled with I-line compatible pellicles. Similarly, deep ultraviolet (DUV) equipment requires DUV compatible pellicles. Although DUV pellicles can be used in I-line applications, the cost of DUV pellicles greatly outweighs the benefit of this generic feature. Conversely, inexpensive I-line membrane pellicles used at DUV wavelengths will rapidly degrade and ultimately corrupt the printed image on a wafer.
Currently, semiconductor manufacturers attempt to manage problems associated with a mixed wavelength photomask manufacturing process by using complex part numbers on each photomask component. A technician must generally read and cross-reference each part number with, for example, a list of part numbers to determine if a pellicle is compatible with a specific manufacturing process. This technique suffers from several opportunities for human error. Any lapse in concentration or error in the part number list may result in the improper combining of photomask assemblies for a manufacturing process, which may cause costly mistakes.
Further, a conventional manufacturing method uses part numbers that are based on a particular photomask component. For example, a part number on a pellicle may use a four-digit number to determine the type of material used to make a pellicle film. This part number must then be cross-referenced to determine which exposure wavelengths are compatible for this type of material. However, a photomask may use a five-digit part number to define which exposure wavelengths are compatible with the photomask. Using this different part numbering system for each photomask component may result in improper combining of photomask assemblies for a manufacturing process, which may further cause costly mistakes.